Once little understood, a liver ailment known as hepatitis C is capturing the concern of doctors at Johns Hopkins Hospital following a study showing that 18 percent of patients tested in the emergency room carried the potentially deadly virus.

The high rate of hepatitis C, called "astounding" by one emergency room physician, came to light when physicians re-examined frozen blood samples that had been collected from patients in 1988 for another study charting the prevalence of the human immunodeficiency virus, which causes AIDS.

Hopkins plans to survey a new sample of patients in the summer to see if the infection has since increased or declined.

"HIV is already known to be ravaging certain minority populations," said Dr. Gabor Kelen, acting director of emergency medicine at Hopkins. "What is particularly alarming are the extraordinary rates of hepatitis C virus among minorities in the population."

For instance, half of the black men between the ages of 25 and 44 were infected with hepatitis C. And about 60 percent of the men in that group tested positive for the AIDS virus, hepatitis C, or its cousin, hepatitis B.

Details of the study appear in this week's New England Journal of Medicine.

Hepatitis C is a liver infection that may initially cause fever and jaundice. While half of all people contracting the virus will not suffer any long-term problems, the other half develop a chronic disease in which the liver gradually loses its abilility to perform life-sustaining functions.

Until the late 1980s, doctors did not know what caused the ailment. They simply observed patients who suffered from a mysterious liver disease that wasn't hepatitis B but behaved in similar ways.

In May 1990, the American Red Cross and other blood banks started using a new test to screen all donated blood for hepatitis C. Greater awareness of the virus prompted the Hopkins team to take a new look at its collection of stored blood samples, which had already been tested for the virus that causes acquired immune deficiency syndrome.

Physicians looked at blood samples taken from 2,523 emergency room patients over the age of 15 who needed to have blood drawn for other purposes related to their treatment.

One-quarter of the patients tested in the Hopkins study carried either of the two hepatitis strains or HIV. Considered separately, 5 percent carried hepatitis B, 6 percent carried HIV while 18 percent were infected with hepatitis C.

Dr. Kelen said the study gives a glimpse of the serious problem facing inner-city Baltimore. People needing emergency attention are generally a sicker population who are more likely to have a host of illnesses besides the ones for which they are being treated.

The study also is a graphic reminder that health-care workers should observe the "universal precautions" against acquiring blood-borne infections, Dr. Kelen said. These include the wearing of plastic face shields, surgical gloves and gowns to ward off transmission from open cuts and blood splashes.